1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer or a copying machine for producing a copy and a management system therefor which can prevent illegal copying.
2. Description of the Prior Art
Recently, copying machines are developed and used widely. Then, infringement of copyrights and illegal copies of secret documents become problems to be solved. However, it is impossible for analog copying machines to check an original document precisely. Further, for a document including a partial copy, it is quite impossible to manage hard copies after the second generation copy of the original document.
There have been proposed many apparatuses and methods in order to prevent the leakage of secret documents by copying. For example, if a copying apparatus detects a phrase registered beforehand such as "DO NOT COPY" or "CONFIDENTIAL", the copy operation is stopped (Japanese Patent laid open Publication 266,759/1990). However, making an illegal copy is easy by shading the registered phrases. Therefore, if a copy is once obtained by shading the registered phrases, it cannot be prevented to be leaked. It is also disclosed that an illegal copy can be detected by using information on the characteristics of a document on the image of the whole document such as the position and the number of a specified character such as "u". However, this requires a memory of large capacity to store all characters.
In order to prevent illegal copies of paper moneys and valuable securities with a copying apparatus, it is proposed to store a pattern of characteristics of an image for prohibiting copying and to stop the copying operation for an image of a document including such a characteristic (Japanese Patent laid open Publication 52,384/1990). If a whole document image is stored as a characteristic pattern, this copying machine can deal with even a case where a part of the characteristic pattern is shaded. However, in order to store all characteristic information for a secret document, a memory of large capacity is needed also in this case. Further, it is difficult to detect all information in correspondence to copying operation.
It is also proposed that a password is needed for a copying apparatus (Japanese Patent laid open Publications 120,753/1990 and 120,561/1991). However, it is a problem that a password is needed even for a non-secret document.
On the other hand, in order to trace the origin of leakage of a secret document, it is proposed to add information to a part of a copy of a document (Japanese Patent laid open Publication 111,161/1990). That is, information such as recognition data of a copying apparatus, copy data and copy time are expressed as bar codes, and the bar codes are added in a part of a copy. However, this approach is ineffective if the bar codes to be added in a copy is shaded or if bar codes added for another non-secret document is stuck to a copy to re-generate the copy.
It is also proposed to change a font for a secret document from that of an ordinary document in order to recognize a copy of secret document from the others (U.S. Pat. No. 4,837,737). Then, even if the copies of secret documents are edited or changed, a copy of a secret document can be recognized. However, this approach is ineffective if a document includes only drawings, graphs or the like.
The synthesis of character data with image data in the image processing with a computer is proposed (Yasuhiro Nakamura and Kineo Matsui, "A unified coding method of image and text data using three-level micro-patterns", Gazou Densi Gakkaishi Vol. 17, No.1 (1988) 3-9). In the image processing wherein the sum of the dots in a small area of n.times.n dots is used as a density of a dot to be displayed, a pattern corresponds with a density, and text data are synthesized with image data by using the patterns. However, if this approach is applied to a copying machine, it is impossible to recognize the patterns in a hard copy. Therefore, this approach cannot be applied to trace the leakage of secret documents.